Lead-acid batteries have long been used as a power source for automotive starting, vehicular traction, emergency lighting, powering portable tools and appliances, and standby power. Additionally, these batteries are believed to be useful in improving the economics of power generation via load levelling or peak power shaving (i.e., storage of power generated during off-peak hours to supplement that generated during times of peak demand). However, economic load-levelling storage batteries require large cells of high capacity, a long life in cycling service, high quality assurance and a minimum of maintenance. Also, economic recovery and recycling of the large quantity of raw materials involved is such batteries is highly desirable.
Lead-acid batteries suitable for load-levelling service are normally classified as either "flat-plate" or "tubular." In each case, the battery consists of a number of individual cells, each of which contains one or more positive electrodes and one or more negative electrodes which are immersed in a sulfuric acid electrolyte and electrically insulated from one another by an acid-resistant porous separator. All positive electrodes within a cell are electrically connected in parallel as are all negative electrodes within a cell. The capacity of each cell is controlled by the amount of positive and negative active material in the cell and by the number of electrodes and the length, width, and/or thickness of such electrodes. The rate at which a cell discharges is controlled by the surface area of the electrodes available for reaction and can be changed by varying the number of electrodes and/or the surface area of each electrode within a cell.
One method of increasing the surface area of battery electrodes, disclosed in U.S. patent application No. 315,722 is profiling the paste layer on each side of the electrode. An ideal deep-cycling storage battery is one in which both the amount of positive and negative active materials and the surface area of the positive and negative plates can be maximized within a given cell volume.
Typically, large flat-plate cells and batteries used in deep-cycle service, include positive and negative plates positioned such that each pasted positive plate side surface faces, and is oriented essentially parallel to, a pasted negative plate side surface of equivalent area. For the positive and negative plates, the preferred pastes are comprised of lead oxide, water, sulfuric acid and binder as the positive paste composition and lead oxide, water, sulfuric acid, binder and expander as the negative plate composition. The plates are normally separated from one another by one or more layers of porous electrically insulating materials, typically made from glass fibers and porous polymeric materials. In one such cell construction, described in U.S. Pat. No. 4,447,508, the positive plate is separated from the negative plate by three layers of insulating material, one of which is wrapped around the positive plate, and one layer of insulting retainer material. This type of battery involves both high labor cost and high material costs. Additionally, it is difficult to increase the capacity of flat-plate cycling batteries by increasing the size or number of plates because of plate growth, a prevalent cause of failure in prolonged cycle service.
In large tubular-type cells and batteries used for deep-cycle service, the positive electrodes normally consist of a one-piece cast lead conductor in the form of a series of parallel spines projecting downward from a single connecting strap, porous sleeves which are closed at the bottom and positioned relative to said spines in a manner which results in an annular space between said spine and said sleeve along the entire length of the spine, and loose positive active particulate material which is vibrated into the annular space. The negative electrode is normally of the pasted flat-plate type described above. -t is typically positioned such that a sole pasted surface of the negative electrode faces and is parallel to a sole positive electrode surface of equivalent area, and spaced apart from the outer surface of the positive electrode by a porous polymeric ribbed sheet separator. As in the case of flat-plate cells, tubular-type cells consisting of a plurality of positive electrodes, a plurality of negative electrodes and a plurality of complex separators, are difficult to manufacture and inherently costly.
To overcome some of these problems, it is now possible to process lead-acid battery plate stock in continuous form. It is known that continuous grid casting, as described in U.S. Pat. Nos. 4,349,067 and 4,415,010, and continuous metal expansion, as described in U.S. Pat. No. 3,853,626, can be used to produce continuous lengths of battery grid stock which can be passed directly into a continuous pasting machine, such as that described in U.S. Pat. No. 4,271,586. The stock exiting the continuous paster is normally cut into individual grids, but it may be cured in coil from, as disclosed in pending U.S. patent application No. 315,722 and electrochemically formed prior to being divided into individual plats, as disclosed in pending U.S. patent application No. 361,029.
One cell construction utilizing pasted plate stock in continuous form is described in U.S. Pat. No. 3,862,861. It incorporates a continuous length of positive plate stock and a continuous length of negative plate stock positioned such that the pasted positive plate surface is opposite and parallel to the pasted negative plate surface and separated therefrom by continuous separator material. The cell is produced by coiling the aforesaid three-piece composite to form a "jelly-roll" structure which is then placed in a battery container.
A major problem inherent in both large flat plate batteries and a tubular batteries is stratification of the sulfuric acid electrolyte, i.e., the tendency for the electrolyte to separate into a layer of high specific gravity at the bottom of the cell and a layer of low specific gravity at the top of the cell. The high gravity layer at the bottom of the cell accelerates sulfation of the negative plate which reduces cell capacity, and the low gravity layer at the top of the cell increases the tendency towards corrosion of the positive grid which increases the electrical resistance in the cell and, eventually, results in failure of the plate. Acid stratification becomes more pronounced as the height of the cell increases.
One commonly used method to overcome acid stratification is bubbling air through the electrolyte; however, this is costly. Another commonly used method is overcharging the cell to generate gas which mixes the electrolyte, but this technique increases electrolyte loss and maintenance costs and shortens cell life.
Recombination of the gases generated during charging is also a widely used means of minimizing, or eliminating, electrolyte loss. One method of accomplishing such recombination in absorbed electrolyte cells containing flat vertical plate electrodes is described in U.S. Pat. Nos. 4,401,730 and 4,119,772 and involves the reaction of oxygen gas generated at the positive electrode with excess negative active material which is not covered by electrolyte. A similar approach is described in U.S. Pat. No. 4,425,412, which discloses a cell with improved deep-cycle capability based upon a plurality of horizontally disposed positive plates, a plurality of horizontally disposed negative plates, and free unabsorbed electrolyte, in which all active plate surfaces are submerged in electrolyte except the entire uppermost pasted surface of the top negative plate which is free to take part in the recombination reaction. The benefits obtained from the horizontal disposition of the battery plates described in U.S. Pat. No. 4,425,412, are limited to small cells, because in cells containing large plates, the movement of gas and electrolyte within the cell is restricted. Recombination can also be accomplished by the incorporation of a separate non-lead catalyst, as described in U.S. Pat. No. 3,470,024. Both the incorporation of excess negative active material and the use of separate catalysts, however, lower the cell efficiency.
Another factor which reduces the efficiency of large power cells is the increased electrical resistance resulting from the small size and off-center locations of the plate lugs which connect the battery plate to the external electrical contacts of the cell. As illustratively shown in U.S. Pat. Nos. 4,742,611 and 4,509,253, these plate lugs are situated towards one end of the plate and have a cross-sectional area parallel to the upper border of the plate that is normally less than 15% of the cross-sectional area of the upper border when that border is sectioned along the length of the battery plate. The off-center location and reduced cross-sectional area of the lug increases the length of the current-carrying path and, thus, the internal resistance of the cell.
A 2,000-volt, 10 megawatt lead-acid battery energy storage system has been developed utilizing 8,256 cells arranged in 8 parallel strings, each containing 1,032 cells connected in series. The battery includes provision to supply compressed air to each of the individual cells in order to circulate the electrolyte as a means of avoiding acid stratification. It also includes a 7,500-gallon water storage tank and distribution lines to each cell. In operation, the system requires 14 mandays of labor twice per year to water the cells, and utilizes approximately 2,000 tons of lead which must be recycled at the end of life.
While lead-acid storage batteries are suitable for deep-cycling service requirements, there are many limitations. Accordingly, it is an object of the present invention to provide an improved battery structure which reduces or eliminates many of the problems normally associated with the prior art lead-acid storage batteries.